IIS site opening, stops after login pages because app pool stopped

We got a call from customer, they Newly hosted the site and the site was loading fine for few seconds/Minutes. then default Application pool was stopped.

Solutions:

We have checked and comes to know, customer using default application pool, but they are not set the .Net version in the respective application pool.

We couldn't find any windows event logs for this issue.

After mapped the .net as .net 4.5, site starts works continuously with any issue.

Note:
Here we fix the issue after mapping the .Net 4.5 version in application pool. But for you may be facing issue because of other reason.